An adhesive composition which serves to adhere a biomedical electrode to skin and to establish an electrical connection between skin and an electrical medical apparatus desirably has multiple characteristics which are difficult to achieve in one composition. The composition should have the characteristics of a good medical adhesive and those of a good electrical conductor.
A good medical pressure-sensitive adhesive needs to be dermally non-irritating, sufficiently plastic and tacky to wet skin and adhere, and sufficiently cohesive both to resist shear and peel forces and to pull away from skin without leaving a residue. Compositions which make good medical adhesives are well known. Exemplary classes are polyvinyl ethers and copolymers of acrylic acid and isooctylacrylate. Copolymers used on many commercially available medical tapes include adhesives made from isooctylacrylate and up to ten percent by weight acrylic acid or N-vinylpyrrolidone. An exemplary formulation is a copolymer of 96% isooctylacrylate and 4% acrylic acid; another adhesive is based on 91% isooctylacrylate and 9% N-vinylpyrrolidone. While these formulations make excellent medical adhesives, they are not suitable for electrically-conductive adhesives. Conductive adhesives require a substantial amount of plasticizing electrically-conductive liquid. The plasticizing electrically-conductive liquid is polar and usually has ionic species in solution. The isooctylacrylate and acrylic acid or N-vinylpyrrolidone copolymers are not crosslinked and are not soluble in water and other polar liquids. If they are mixed with polar liquids and ionic solutions, phase separation results.
The electrical properties desired of a conductive, pressure-sensitive adhesive relate to its ability to transmit ionic electric current to or from the body and, in co-operation with a conductor, to convert ionic current to electric current or electric current to ionic current. The ability of the compositon to accurately transmit the ionic electric current generated by biomedical events is very important. To optimize electrical performance, a low impedance at the interface of the composition and the skin is desirable. Also important is the ability of the biomedical electrode to translate ionic current to electrical current and electrical current to ionic current. In many instances this translation is effected by means of an electrochemical reaction. For example, in a classic silver/silver chloride system the conductor is a layer of metallic silver having a thin layer of silver chloride on its surface, and the electrolyte medium (conductive gel or conductive adhesive) contains chloride ions. The electrical performance of this system depends in part on the concentration and mobility of ionic species in the composition. The hydrogen ion concentration is also relevant. In other systems, the translation may be effected by capacitive coupling. In that circumstance, the charge density of the electrode is important to the ability of the electrode to translate the signals encountered. Here again, the concentration of ionic species is important to good performance. For example, in a patient grounding plate large alternating currents (typically 0.1-1A at about 500 KHz) are encountered. In these conditions, capacitive coupling is possible and the presence of ionic species facilitates transfer.
Conductive pressure-sensitive adhesives are shown in many patents. U.S. Pat. No. 4,274,420 to Hymes, U.S. Pat. No. 4,352,359 to Larimore, and U.S. Pat. Nos. 4,524,087, 4,539,996, and 4,554,924 to Engel are exemplary. Two of the Engel Patents (U.S. Pat. Nos. 4,524,087 and 4,539,996) disclose an electrically-conductive adhesive formed by an essentially solventless free radical polymerization of an adhesive precursor having a polyhydric alcohol, at least on ionic monomer, a crosslinker, and an initiator. Non-ionic comonomers may be included. Ionic monomers listed are salts of .alpha.,.beta.-unsaturated carboxylic acids such as potassium and sodium acrylate and sodium methacrylate. Non-ionic comonomers listed are acrylic acid, methacrylic acid, and hydroxyethyl methacrylate.
The third Engel patent (U.S. Pat. No. 4,554,924) discloses a conductive adhesive formed by an essentially solventless free radical polymerization of an adhesive precursor having a polyhydric alcohol, at least one non-ionic monomer, an initiator, a crosslinker, and an ionizable salt present in an amount sufficient to render the composition conductive. Non-ionic monomers shown are acrylic acid, methacrylic acid, hydroxyethyl methacrylate, and N-vinylpyrrolidone. The specification states that the non-ionic polymerization material may comprise one non-ionic monomer or a mixture of non-ionic monomers. Also, it says ionic monomers may be included.
Conductive adhesives made according to the Engel patents are used in patient grounding plates, transcutaneous electrical nerve stimulation (TENS) electrodes, and diagnostic electrocardiogram (EKG/ECG) electrodes. While these adhesives provide excellent adhesive and electrical properties for some applications, optimizing the electrical properties without adversely affecting adhesion properties has not proved possible. Experience has shown that increasing the water content of the Engle adhesive compositions to about 25% improves the electrical performance of electrodes coated with the adhesives. The reason for this empirical observation is not known; one possible explanation is that increased water facilitates wetting of skin, thereby reducing skin impedance. Unfortunately, increasing water content to optimum levels for electrical performance is found to decrease the initial tack and cohesive strength of the polymer, resulting in poor skin adhesion and residue left on the skin when the electrode is removed. While tackifiers may be used, they have not been found particularly helpful in optimizing the electrical and adhesive properties.
Another conductive adhesive is disclosed in U.K. Patent Appln. 2,115,431 (published Sept. 7, 1983). That publication describes adhesives formed by dissolving or dispersing polymers in a plasticizing liquid and subjecting the mixture to radiation energies at least equivalent to 100,000 electron volts (X-ray, gamma and beta ray, and electron beam irradiation). The conductive adhesive described is prepared from polyvinylpyrrolidone, polyethylene glycol, magnesium acetate, methyl paraben, propyl paraben, FD&C Blue #2 and water.
A commercially electrically-conductive adhesive sold under the trademark Polyhesive.TM. (Valleylab, Inc., Boulder, CO) is believed to be made according to the disclosure of U.K.. Appn. 2,115,431. That adhesive exhibits good electrical properties but performs poorly as a biomedical adhesive. The initial adhesion to skin is not strong enough for many applications and the adhesive tends to lose moisture with time resulting in loss of adhesion at the edges of an electrode.
The conductive adhesives described above are crosslinked swellable polymeric matrices. When a polymeric matrix has sufficient crosslinking it can be fully swelled without dissolving. When these crosslinked materials are swelled with water they are called hydrogels. A non-adhesive hydrogel used for contact lenses is disclosed in U.K. Patent Appl. 2,130,743 (published June 6, 1984) and may be made as a terpolymer of a monomer mixture comprising N-vinylpyrrolidone, a short chain alkyl acrylate or methacrylate, and a short chain unsaturated carboxylic acid. That application requires the N-vinylpyrrolidone and the ester to make up the majority of the monomer mixture. It recommends the acid monomer not exceed 10% of the monomer mixture. The examples all have acid monomers comprising less than 4% (by weight) of the total monomers and less than 5% (by weight) of the N-vinylpyrrolidone. U.S. Pat. No. 4,451,630 shows another terpolymer used for contact lenses having N-vinylpyrrolidone and a short chain unsaturated acrylic acid. The monomer mixture for that terpolymer is N-vinylpyrrolidone (25 to 50% by weight) methacrylic or acrylic acid (up to 5% by weight), a triazine crosslinker (0.1 to 1% by weight), and hydroxy alkyl acrylate or methacrylate (balance). The examples have acid monomer comprising less than 2% by weight of the total monomer mixture and less than 5% by weight of the N-vinylpyrrolidone.
A class of adhesives that can be dispersed or dissolved in polar solutions including water is the class of repulpable adhesives. Theu are generally not crosslinked. One such repulpable adhesive is made with a comonomer of butyl acrylate and methyl diethanol ammonium acrylate. While ionic species may be somewhat soluble in these adhesives sufficient ionic concentration and mobility for optimal electrical performance has not been possible when the total water content is low enough to avoid dispersion of the polymers.
Another repulpable adhesive is soluble in both acid and basic solutions. Japanese Appl. No. 80-18772 (published May 17, 1980) describes a non-conductive adhesive that has a copolymer made from a polymerizable vinyl carboxyl contaning monomer and a vinylpyrrolidone, vinylpyridine or other basic monomer. The copolymer contains acid functional groups and alkaline functional groups in a prescribed ratio of 60-99.5% acid monomer to 40-0.5% basic monomer. The adhesive contains a water soluble plasticizer such as a polyhydric alcohol.
Mixtures of polymers rich in hydrogen bond donating groups with polymers rich in hydrogen bond accepting sites are known to be insoluble in water. For example, polyacrylic acid, a polymer rich in hydrogen bond donating sites, and polyvinylpyrrolidone, a polymer rich in hydrogen bond accepting sites, are each soluble in water. When solutions containing the two polymers are mixed, a partially swelled matrix rapidly precipitates out of solution.